It is very difficult to starve an engine of oxygen, even if you have a relatively air tight enclosure. Best approach is to have a small opening between the engine compartment and the lazarette or if not practical, just install a vent at the stern. You do not need to force air through it. The logic is that when gaseous concentrations change, the gas molecules travel super fast to equalize the concentrations (feet per second). A better way to explain it is that when you fart, the smell quickly fills the room, through the clothes and without any blowers. Exactly the same happens if the oxygen concentration in the engine compartment drops... oxygen molecules from any adjacent spaces quickly find their way through the openings. A few square inches is enough, typically towards the lazarette. You do not need to exchange all the nitrogen, etc.
Ventilation makes more sense if you want to evacuate gaseous vapors quickly before they spread as is necessary with gas engines. It is also very hard and inefficient to use ventilation to lower the temperature (water based heat exchangers are much more efficient in heat transfer). Consider a car on the highway, where the engine makes probably 30-40 hp to drive at highway speeds, then the volume of air and the speed at which it hits the radiator and then compare that mass transfer to the tiny 3" fan. It is basically of no use.
Another factor is if you just want to cool the alternator (a much easier task) but again, if you think about the mass volume/temperature difference vs. the heat generated by the alternator... it is hard to believe significant heat exchange occurs. A better way would be to blow cold air towards the alternator or consider liquid cooling but that is complicated.
In summary, unless you plan to make your compartment air-tight, I would not worry about it and spare the effort. If for some reason your engine dies out of oxygen starvation then you can reconsider but I think it is highly unlikely.